Transmission of data using optical carriers enables very high bandwidths and numbers of multiplexed channels with low signal loss and distortion. A coherent laser light beam is amplitude modulated with a data signal, and propagates to a remote receiver via a system of optical fibres and repeaters. The light beam may advantageously be modulated with electrical signals in the microwave frequency range using an electro-optic modulator such as a Mach-Zehnder modulator or optical coupler.
Mach-Zehnder electro-optic modulators are well known. An electro-optic modulator based on a Mach-Zehnder interferometer generally includes a monolithic substrate formed of an electro-optic material such as LiNbO3 or InP. An optical waveguide is formed in the substrate having two arms or branches which extend generally in parallel with each other. The effective index of refraction of the material in the waveguide is higher than the index of refraction of the material of the substrate.
In the absence of an applied electrical bias voltage, an input optical or light beam produced by a laser or the like applied to the waveguide divides between the branches. The optical signals propagating through the branches recombine at the optical output of the waveguide. If the optical path lengths of the branches are equal, or differ by an integral number of wavelengths, then the optical signals recombine in phase with each other, such that their amplitudes are additive and an optical output signal which is essentially similar to the optical input signal appears at the output of the waveguide.
If the optical path lengths of the branches differ by a half integral number of wavelengths, the optical signals emerging from the branches are 180° out of phase with each other. The amplitudes of the signals combine subtractively, cancelling each other out, such that a zero output is produced at the optical output.
Application of a predetermined electrical bias voltage differential to one branch of the waveguide relative to the other branch causes the complex indices of refraction of the material in the branches to vary differently due to the electro-optic effect, such that the effective optical lengths (and absorption) of the branches vary accordingly. At a bias voltage known in the art as Vπ, the effective optical lengths have varied to such an extent that the optical signals emerging from the branches are 180° out of phase compared to the situation when no bias voltage is applied. If the path lengths of an unbiased modulator are the same, then at a bias voltage of Vπ the optical signals will interfere destructively and cancel each other out, such that a zero output is produced at the optical output. If the path lengths of an unbiased modulator differ by a half integral number of wavelengths, then at a bias voltage of Vπ the optical signals will interfere constructively so that the optical output signal is essentially similar to the optical input signal. An electrical data signal, normally in the microwave frequency range, is applied to one or both of the arms. The optical carrier signal exiting the device is thus modulated by the data signal
For most optical communication applications, it is desirable to bias the modulator at a voltage Vπ/2. However, device instabilities and environmental effects, especially temperature variations, cause the operating point to drift over time, and constant readjustment is required to maintain the proper operating point. The bias point must be maintained during operation to achieve maximum dynamic range, since second order harmonic distortion increases rapidly with increasing bias voltage error.
The bias point is generally maintained by providing a low frequency (˜400 Hz) sinusoidal pilot tone to the bias voltage or to the data signal. This causes a further low frequency modulation in the output signal, whose phase and amplitude is determined by the distance from the bias point. The output arm of the modulator includes a tap to a photodetector, enabling the output signal to be monitored. The output signal is compared with the pilot tone using a phase sensitive detector, and this enables the use of a feedback loop to maintain the bias point. Examples of modulators employing such a system are described in U.S. Pat. No. 5,003,624 (in which the pilot tone is applied to the bias voltage) and U.S. Pat. No. 5,170,274 (in which the pilot tone is applied to the data signal).
A known electro-optic modulator system including an automatic bias adjustment is illustrated in FIG. 1, and generally designated 10. A laser 12 feeds a coherent light beam through an optical fibre 14 into an optical input 16 of a Mach-Zehnder modulator 18, optical coupler, or other appropriate electro-optic modulator. The light beam propagates through a waveguide having two branches 20 and 22, which recombine at an optical output 24 of the modulator 18. An electrical data signal, preferably in the microwave frequency range, is applied to the branches 20, 22 via a driver 25 and modulation signal “T” inputs 26, 27. A bias voltage is also applied to the branches 20, 22 via the “T” inputs 26, 27. The optical carrier signal constituted by the laser beam is modulated with the data signal, and fed through an optical fibre 28 to a remote receiver (not shown). An amplitude modulation (AM) pilot tone source 29 is applied to the driver of the data signal.
A tap 30 is provided in the output optical fibre 28 which leads through an optical fibre pigtail 32 to a photodetector 34. The output of the photodetector 34 and the pilot tone modulating signal 29 are applied to a phase sensitive detector 36, which compares the low frequency modulation of the output signal with the pilot tone 29. Depending on the phase and amplitude of the modulation on the output signal, the bias voltage of the lower arm 20 of the modulator is adjusted to maintain the bias point.
This system works well but the tapping of some of the output signal to a photodetector represents optical loss. The overall transmitted power is therefore reduced. Furthermore, the generation of a sinusoidal pilot tone requires hardware which occupies valuable space near the modulator. The feedback loop also requires a phase sensitive detector and DC coupled amplifiers which are expensive and again occupy valuable space.